SYNOPSIS

DESCRIPTION

getrlimit()
and
setrlimit()
get and set resource limits respectively.
Each resource has an associated soft and hard limit, as defined by the
rlimit
structure (the
rlim
argument to both
getrlimit()
and
setrlimit()):

The soft limit is the value that the kernel enforces for the
corresponding resource.
The hard limit acts as a ceiling for the soft limit:
an unprivileged process may only set its soft limit to a value in the
range from 0 up to the hard limit, and (irreversibly) lower its hard limit.
A privileged process (under Linux: one with the
CAP_SYS_RESOURCE
capability) may make arbitrary changes to either limit value.

The value
RLIM_INFINITY
denotes no limit on a resource (both in the structure returned by
getrlimit()
and in the structure passed to
setrlimit()).

resource
must be one of:

RLIMIT_AS

The maximum size of the process's virtual memory (address space) in bytes.
This limit affects calls to
brk(2),
mmap(2)
and
mremap(2),
which fail with the error
ENOMEM
upon exceeding this limit.
Also automatic stack expansion will fail
(and generate a
SIGSEGV
that kills the process if no alternate stack
has been made available via
sigaltstack(2)).
Since the value is a long, on machines with a 32-bit long
either this limit is at most 2 GiB, or this resource is unlimited.

RLIMIT_CORE

Maximum size of
core
file.
When 0 no core dump files are created.
When nonzero, larger dumps are truncated to this size.

RLIMIT_CPU

CPU time limit in seconds.
When the process reaches the soft limit, it is sent a
SIGXCPU
signal.
The default action for this signal is to terminate the process.
However, the signal can be caught, and the handler can return control to
the main program.
If the process continues to consume CPU time, it will be sent
SIGXCPU
once per second until the hard limit is reached, at which time
it is sent
SIGKILL.
(This latter point describes Linux 2.2 through 2.6 behavior.
Implementations vary in how they treat processes which continue to
consume CPU time after reaching the soft limit.
Portable applications that need to catch this signal should
perform an orderly termination upon first receipt of
SIGXCPU.)

RLIMIT_DATA

The maximum size of the process's data segment (initialized data,
uninitialized data, and heap).
This limit affects calls to
brk(2)
and
sbrk(2),
which fail with the error
ENOMEM
upon encountering the soft limit of this resource.

RLIMIT_FSIZE

The maximum size of files that the process may create.
Attempts to extend a file beyond this limit result in delivery of a
SIGXFSZ
signal.
By default, this signal terminates a process, but a process can
catch this signal instead, in which case the relevant system call (e.g.,
write(2),
truncate(2))
fails with the error
EFBIG.

RLIMIT_LOCKS (Early Linux 2.4 only)

A limit on the combined number of
flock(2)
locks and
fcntl(2)
leases that this process may establish.

RLIMIT_MEMLOCK

The maximum number of bytes of memory that may be locked
into RAM.
In effect this limit is rounded down to the nearest multiple
of the system page size.
This limit affects
mlock(2)
and
mlockall(2)
and the
mmap(2)
MAP_LOCKED
operation.
Since Linux 2.6.9 it also affects the
shmctl(2)
SHM_LOCK
operation, where it sets a maximum on the total bytes in
shared memory segments (see
shmget(2))
that may be locked by the real user ID of the calling process.
The
shmctl(2)
SHM_LOCK
locks are accounted for separately from the per-process memory
locks established by
mlock(2),
mlockall(2),
and
mmap(2)
MAP_LOCKED;
a process can lock bytes up to this limit in each of these
two categories.
In Linux kernels before 2.6.9, this limit controlled the amount of
memory that could be locked by a privileged process.
Since Linux 2.6.9, no limits are placed on the amount of memory
that a privileged process may lock, and this limit instead governs
the amount of memory that an unprivileged process may lock.

RLIMIT_MSGQUEUE (Since Linux 2.6.8)

Specifies the limit on the number of bytes that can be allocated
for POSIX message queues for the real user ID of the calling process.
This limit is enforced for
mq_open(3).
Each message queue that the user creates counts (until it is removed)
against this limit according to the formula:

where
attr
is the
mq_attr
structure specified as the fourth argument to
mq_open(3).

The first addend in the formula, which includes
sizeof(struct msg_msg *)
(4 bytes on Linux/i386), ensures that the user cannot
create an unlimited number of zero-length messages (such messages
nevertheless each consume some system memory for bookkeeping overhead).

RLIMIT_NICE (since Linux 2.6.12, but see BUGS below)

Specifies a ceiling to which the process's nice value can be raised using
setpriority(2)
or
nice(2).
The actual ceiling for the nice value is calculated as
20 - rlim_cur.
(This strangeness occurs because negative numbers cannot be specified
as resource limit values, since they typically have special meanings.
For example,
RLIM_INFINITY
typically is the same as -1.)

RLIMIT_NOFILE

Specifies a value one greater than the maximum file descriptor number
that can be opened by this process.
Attempts
(open(2),
pipe(2),
dup(2),
etc.)
to exceed this limit yield the error
EMFILE.
(Historically, this limit was named
RLIMIT_OFILE
on BSD.)

RLIMIT_NPROC

The maximum number of processes (or, more precisely on Linux, threads)
that can be created for the real user ID of the calling process.
Upon encountering this limit,
fork(2)
fails with the error
EAGAIN.

RLIMIT_RSS

Specifies the limit (in pages) of the process's resident set
(the number of virtual pages resident in RAM).
This limit only has effect in Linux 2.4.x, x < 30, and there only
affects calls to
madvise(2)
specifying
MADV_WILLNEED.

Specifies a limit (in microseconds)
on the amount of CPU time that a process scheduled
under a real-time scheduling policy may consume without making a blocking
system call.
For the purpose of this limit,
each time a process makes a blocking system call,
the count of its consumed CPU time is reset to zero.
The CPU time count is not reset if the process continues trying to
use the CPU but is preempted, its time slice expires, or it calls
sched_yield(2).

Upon reaching the soft limit, the process is sent a
SIGXCPU
signal.
If the process catches or ignores this signal and
continues consuming CPU time, then
SIGXCPU
will be generated once each second until the hard limit is reached,
at which point the process is sent a
SIGKILL
signal.

The intended use of this limit is to stop a runaway
real-time process from locking up the system.

RLIMIT_SIGPENDING (Since Linux 2.6.8)

Specifies the limit on the number of signals
that may be queued for the real user ID of the calling process.
Both standard and real-time signals are counted for the purpose of
checking this limit.
However, the limit is only enforced for
sigqueue(2);
it is always possible to use
kill(2)
to queue one instance of any of the signals that are not already
queued to the process.

RLIMIT_STACK

The maximum size of the process stack, in bytes.
Upon reaching this limit, a
SIGSEGV
signal is generated.
To handle this signal, a process must employ an alternate signal stack
(sigaltstack(2)).

Since Linux 2.6.23,
this limit also determines the amount of space used for the process's
command-line arguments and environment variables; for details, see
execve(2).

RETURN VALUE

On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EFAULT

rlim
points outside the accessible address space.

EINVAL

resource
is not valid;
or, for
setrlimit():
rlim->rlim_cur
was greater than
rlim->rlim_max.

EPERM

An unprivileged process tried to use
setrlimit()
to
increase a soft or hard limit above the current hard limit; the
CAP_SYS_RESOURCE
capability is required to do this.
Or, the process tried to use
setrlimit()
to increase
the soft or hard
RLIMIT_NOFILE
limit above the current kernel
maximum
(NR_OPEN).

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001.
RLIMIT_MEMLOCK
and
RLIMIT_NPROC
derive from BSD and are not specified in POSIX.1-2001;
they are present on the BSDs and Linux, but on few other implementations.
RLIMIT_RSS
derives from BSD and is not specified in POSIX.1-2001;
it is nevertheless present on most implementations.
RLIMIT_MSGQUEUE,
RLIMIT_NICE,
RLIMIT_RTPRIO,
RLIMIT_RTTIME,
and
RLIMIT_SIGPENDING
are Linux-specific.

NOTES

One can set the resource limits of the shell using the built-in
ulimit
command
(limit
in
csh(1)).
The shell's resource limits are inherited by the processes that
it creates to execute commands.

BUGS

In older Linux kernels, the
SIGXCPU
and
SIGKILL
signals delivered when a process encountered the soft and hard
RLIMIT_CPU
limits were delivered one (CPU) second later than they should have been.
This was fixed in kernel 2.6.8.

In 2.6.x kernels before 2.6.17, a
RLIMIT_CPU
limit of 0 is wrongly treated as "no limit" (like
RLIM_INFINITY).
Since Linux 2.6.17, setting a limit of 0 does have an effect,
but is actually treated as a limit of 1 second.

A kernel bug means that
RLIMIT_RTPRIO
does not work in kernel 2.6.12; the problem is fixed in kernel 2.6.13.

In kernel 2.6.12, there was an off-by-one mismatch
between the priority ranges returned by
getpriority(2)
and
RLIMIT_NICE.
This had the effect that actual ceiling for the nice value
was calculated as
19 - rlim_cur.
This was fixed in kernel 2.6.13.

Kernels before 2.4.22 did not diagnose the error
EINVAL
for
setrlimit()
when
rlim->rlim_cur
was greater than
rlim->rlim_max.